Method for manufacturing a hybrid structure

ABSTRACT

A method for manufacturing a hybrid structure comprising an effective layer of piezoelectric material having an effective thickness and disposed on a supporting substrate having a substrate thickness and a thermal expansion coefficient lower than that of the effective layer includes: a) a step of providing a bonded structure comprising a piezoelectric material donor substrate and the supporting substrate, b) a first step of thinning the donor substrate to form a thinned layer having an intermediate thickness and disposed on the supporting substrate, the assembly forming a thinned structure; c) a step of heat treating the thinned structure at an annealing temperature; and d) a second step, after step c), of thinning the thinned layer to form the effective layer. The method also comprises, prior to step b), a step a′) of determining a range of intermediate thicknesses that prevent the thinned structure from being damaged during step c).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase entry under 35 U.S.C. § 371 ofInternational Patent Application PCT/FR2016/052674, filed Oct. 17, 2016,designating the United States of America and published as InternationalPatent Publication WO 2017/068269 A1 on Apr. 27, 2017, which claims thebenefit under Article 8 of the Patent Cooperation Treaty to FrenchPatent Application Serial No. 1559993, filed Oct. 20, 2015.

TECHNICAL FIELD

This application relates to the field of manufacturing hybridstructures, especially structures that comprise a layer of piezoelectricmaterial.

BACKGROUND

In the field of acoustic surface wave or volume devices (respectively,“SAW” for “Surface Acoustic Wave” and “BAW” for “Bulk Acoustic Wave” inthe English terminology), the hetero-structures comprising a layer oflithium tantalate (LiTaO₃) disposed on a silicon substrate have agrowing interest: on the one hand, because they are compatible withstandard microelectronics equipment and methods thanks to their siliconsupport substrate, that offer growth opportunities and lower costs; and,on the other hand, because they have technical advantages, such as lessdependence on temperature of the frequency response of SAW devices asexplained in the article by K. Hashimoto, M. Radota et al., “Recentdevelopment of temperature compensated SAW devices,” IEEE Ultrason.Symp. 2011, pages 79 to 86.

For example, LiTaO₃/Si hetero-structures may be prepared from theassembling through bonding and by molecular bonding of two LiTaO₃ and Sisubstrates, respectively. For the manufacture of acoustic wave deviceson these hetero-structures, it is advantageous to be able to apply atemperature above 200° C., or 250° C., to allow the use of materials andprocesses that ensure good performance of the devices.

The holding of the bonding interface between the LiTaO₃ layer and the Sisupport substrate is one of the important factors that manage the goodmechanical strength of the structure in temperature, especially beyond200° C.

It, therefore, appears important to reinforce the energy of theinterface of the hetero-structure prior to the manufacturing steps ofthe acoustic wave device. In the case of a hetero-structure manufacturedby bonding a layer onto a support substrate by molecular adhesion, thebonding interface may, in particular, be reinforced by applying a heattreatment in a temperature range around 200° C. to 300° C. There is,therefore, the issue of applying such heat treatment to thehetero-structure without damaging it because of the significantdifference in coefficient of thermal expansion (CTE for “Coefficient ofThermal Expansion” according to the English terminology) of bothmaterials.

On the other hand, when a hetero-structure with a very thin layer ofLiTaO₃ is required (for example, for the manufacture of volume acousticwave devices), one solution is to transfer the layer using the SMARTCUT® method, including a fragile plane buried in a LiTaO₃ donorsubstrate by introducing light atomic species such as hydrogen orhelium, direct bonding (by molecular adhesion) of this donor substrateonto a silicon support substrate, and detachment at the level of thefragile plane buried so as to transfer a superficial layer of LiTaO₃ onSi. It is known that the surface layer after transfer still has defectsand light atomic species in its thickness. It is, therefore,advantageous to cure this layer by performing an annealing in a suitabletemperature range to allow curing of defects and the evacuation of thelight atomic species, but without damaging the qualities of the thinlayer transferred or the mechanical strength of the hetero-structure.For example, for a layer of LiTaO₃, the suitable temperature range isbetween 400° C. and 600° C.

The issue again is that the LiTaO₃/Si hetero-structures, given the verylarge difference in coefficient of thermal expansion between the twomaterials, hardly support these high thermal budgets.

During the manufacture of hybrid structures or hetero-structures, it iscustomary to carry out heat treatments having a high temperature whenthe surface layer disposed on the support substrate is as thin aspossible, so as to limit stresses and deformations (as can be noticed ina curvature of the substrate) in the hybrid structure. For instance, onecan mention hybrid structures of silicon type on solid silica or siliconon sapphire, which can withstand annealing to around 850° C. when thethickness of the silicon surface layer is less than about 100 nm, andthe thickness of the massive silica substrate is about 700 microns,without suffering prohibitive damage. For higher thicknesses of thesurface layer, typically 1 micron, the maximum applicable temperaturewithout damage decreases, for example, to around 600° C. For even muchhigher thicknesses of the upper layer, for example, 700 microns, themaximum applicable temperature without damage decreases, for example,around 100° C.-150° C.

For a hybrid structure composed of a layer of LiTaO₃ (for example, 10microns thick) disposed on a silicon substrate (for example, 150 mm indiameter and 625 microns thick), the Applicant used this knowledge ofstate of the art: it applied the annealing temperature required toreinforce the bonding interface (i.e., 250° C.) to the final hybridstructure (10 microns LiTaO₃ over 625 microns Si), that is, with thefinest possible surface layer. The Applicant then observed unexpectedresults: a significant degradation of the layer by a so-called“buckling” phenomenon corresponding to the local buckling deformation ofthe LiTaO₃ layer, rendering the hybrid structure unusable.

BRIEF SUMMARY

One of the aims of the disclosure is therefore to propose a method ofmanufacturing a hybrid structure and provide a solution to thedisadvantages of the former art. An aim of the disclosure is notably toprovide a method for applying a required heat treatment without damagingthe hybrid structure.

The disclosure describes a method of manufacturing a hybrid structurecomprising an effective layer of piezoelectric material that has aneffective thickness disposed on a support substrate with a supportthickness and a coefficient of thermal expansion less than that of theeffective layer, the method comprising:

-   -   a) a step of providing a bonded structure comprising a donor        substrate of piezoelectric material and the support substrate,        the bonded structure having a bonding interface between these        two substrates;    -   b) a first step of thinning the donor substrate to form a        thinner layer, having an intermediate thickness, disposed on the        support substrate; the assembly forming a thinned structure;    -   c) a heat treatment step of the thinned structure at an        annealing temperature;    -   d) a second thinning step, after step c), of the thinned layer        to form the effective layer;    -   the method being remarkable in that it comprises, prior to step        b), a step a′) of determining a range of intermediate        thicknesses avoiding degradation of the thinned structure (6′)        during the step c), the range being defined by a threshold        thickness and a maximum thickness, and the intermediate        thickness of the thinned layer being chosen in this range.

The manufacturing method according to the disclosure thus makes itpossible to apply a heat treatment to an annealing temperature requiredto consolidate the bonding interface or to cure all or part of defectspresent in the thinned layer (which will become the effective layer), toa thinned structure for which the range of compatible thicknesses of thethinned layer has been previously determined. The heat treatment isgenerally not applicable to the final hybrid structure, that is, withthe effective layer to its effective thickness, without generatingdamage to the effective layer, especially when unglued areas (bondingdefects or pre-existing engraved patterns on the assembled faces of thesubstrates) are found at the bonding interface according to advantageousfeatures of the disclosure, taken alone or in combination:

-   -   the threshold thickness is determined from a first sensitivity        model whose input parameters include the support thickness, the        thermal expansion coefficients of the donor substrate and of the        support substrate, the annealing temperature and a maximum size        of unglued areas found at the bonding interface of the bonded        structure;    -   the maximum thickness is determined from a second sensitivity        model whose input parameters include the support thickness (of        the support substrate 1), the thermal expansion coefficients of        the donor substrate 2 and the support substrate 1 and the        required annealing temperature in step c) heat treatment;    -   the manufacturing process comprises, after step a′) and prior to        step b), a step a″) of recycling the bonded structure, when the        determining step a′) establishes the upper threshold thickness        at maximum thickness or maximum thickness lower than the        effective thickness;    -   recycling step a″) comprises a separation at the bonding        interface of the bonded structure;    -   recycling step a″) comprises the reuse of the donor and support        substrates resulting from the separation for a new step a) of        providing a bonded structure;    -   the second thinning step d) may further comprise a step of        thinning the support substrate;    -   the effective layer is composed of a material chosen from:        lithium tantalate (LiTaO₃), lithium niobate (LiNbO₃), aluminum        nitride (AlN), zinc oxide (ZnO);    -   the support substrate is composed of a material chosen from the        group: silicon, III-V semiconductors, silicon carbide, glass,        sapphire; and    -   the support substrate comprises one or more surface layers.

The disclosure furthermore relates to a hybrid structure comprising aneffective layer of piezoelectric material with an effective thickness ofless than 50 microns assembled to a support substrate having acoefficient of thermal expansion less than that of the effective layer,the hybrid structure being characterized in that a bonding interfacebetween the effective layer and the support substrate has a bondingenergy greater than 1000 mJ/m² and at least one non-bonded zone whosesize is between 1 and 1000 microns.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the disclosure will emerge fromthe detailed description that follows with reference to the accompanyingdrawings in which:

FIGS. 1a to 1c show a method of manufacturing a hybrid structureaccording to the disclosure;

FIGS. 2a to 2c show three different configurations of thickness ranges,versus the effective thickness, resulting from a determination stepaccording to the disclosure; and

FIG. 3 shows different stages of the manufacturing process according tothe disclosure.

DETAILED DESCRIPTION

The disclosure describes a method for manufacturing a hybrid structure60 comprising an effective layer 20 of piezoelectric material disposedon a support substrate 1 having a support thickness and a coefficient ofthermal expansion less than that of the effective layer 20 (FIG. 1c ).

The method comprises a step a) of providing a bonded structure 6comprising a donor substrate 2 of piezoelectric material and the supportsubstrate 1. Bonded structure 6 has a bonding interface 5 between thesetwo substrates 1, 2 (FIG. 1a ). For example, the donor substrate 2 maybe composed of a material chosen from the group: lithium tantalate(LiTaO₃), lithium niobate (LiNbO₃), aluminum nitride (ALN), zinc oxide(ZnO). The support substrate may be composed of a material selected fromthe group: silicon, III-V semiconductors, silicon carbide, glass,sapphire. One and/or the other of the substrates 1, 2 may optionallycomprise components (all or part of microelectronic circuits) or etchedpatterns on their face to be assembled: the components may be formed ofstacked layers of different natures and having patterns; the etchedpatterns may be formed by cavities intended to ensure a function in thefinal device produced on the hybrid structure 60. The assembly of thebonded structure 6 is advantageously made by molecular bonding betweenthe donor substrate 2 and the support substrate 1. Optionally, anintermediate layer, such as a layer of silicon oxide, silicon nitride orother layer that enhances adhesion molecular bonding, may be added toone or both substrates before assembly. Advantageously, the substrates1, 2 are also subjected to cleaning and/or front surface activationbefore assembly, to enhance the quality and energy of the bondinginterface 5 formed after assembly.

The term “unglued zone” is henceforth called a localized zone at thebonding interface, at which the surfaces of the two substrates are notintimately in contact, with the exception of the unbonded peripheralcrown at the edges of the assembled substrates that form an exclusionzone. An “unbonded area” is typically surrounded by a glued area. Anunglued zone may be linked to a bonding defect, for example, due to thepresence of a particle or other contamination (hydrocarbon or other) atthe bonding interface 5 of the bonded structure 6. It may also berelated to the presence of an etched pattern or local topography on thedonor substrate 2 or the support substrate 1, due to the presence ofcomponents or cavities or laser marks (for example, to ensure thetraceability of the substrates) on their respective faces to beassembled.

After the assembly step, the method comprises a step b) corresponding toa first step of thinning the donor substrate 2 to form a thin layer 2′,with an intermediate thickness, disposed on the support substrate 1; thewhole forming a thinned structure 6′ (FIG. 1b ). The donor substrate 2is thinned at its rear face 4, by techniques of mechanical thinning,mechanical-chemical and/or chemical etching or by a SMART CUT® typeprocess, well known to those skilled in the art. For instance, the donorsubstrate 2 can be thinned by “grinding” (mechanical thinning) and thenby chemical-mechanical polishing. This thinning step precedes a heattreatment step c) the function of which may be to consolidate thebonding interface 5 or to cure defects in the thinned layer 2′ whichbecomes the effective layer 20. It is thus important that the step b)thinning leads to obtaining a thinned structure 6′ that is compatiblewith the heat treatment required in the following step c).

As such, the manufacturing method is remarkable in that it comprises astep a′), prior to step b) of thinning, of determining a range ofintermediate thicknesses for the thinned layer 2′, meant to avoiddegradation of the thinned structure during step c) heat treatment. Therange is defined by a threshold thickness and a maximum thickness, theintermediate thickness of the thinned layer 2′ is thus chosen in thisrange.

The threshold thickness is determined from a first sensitivity modelwhose input parameters include the support thickness (of the supportsubstrate 1), the thermal expansion coefficients of the donor substrate2 and the support substrate 1, the temperature of annealing required instep c) heat treatment and a maximum size of unbonded areas found at thebonding interface 5 of the bonded structure 6.

The first sensitivity model is established from an equation reflectingthe relaxation of a thin layer in compression on a substrate. Aphenomenon of relaxation of a thin layer such as “buckling” requires thepresence of an unglued zone at the interface between the thin layer andthe substrate. The σ_(buckling) critical stress necessary to initiatethe relaxing of the thin layer, which corresponds to the local bucklingdeformation of the layer, can be expressed as:

$\begin{matrix}{\sigma_{buckling} = {\frac{\pi^{2}}{12}\frac{E_{2}}{( {1 - v_{2}^{2}} )}( \frac{h_{2}}{r} )^{2}}} & \{ {{eq}.\mspace{14mu} 1} \}\end{matrix}$with E₂ the Young's modulus of the thin layer, ν₂ the Poisson's ratio ofthe thin layer, h₂ the thickness of the thin layer and r the radius ofthe unbonded zone between the thin layer and the substrate. The stresstranslates a force applied per unit area over the section(length×thickness) of the thin layer. To overcome the length of thesection and to consider only the thickness h₂ of the thin layer, it ispossible to express a critical force normalized by the length,F_(buckling) ^(norm.) in N/m:

$\begin{matrix}{F_{buckling}^{{norm}.} = {{\sigma_{buckling} \cdot h_{2}} = {\frac{\pi^{2}}{12}\frac{E_{2} \cdot h_{2}}{( {1 - v_{2}^{2}} )}( \frac{h_{2}}{r} )^{2}}}} & \{ {{eq}.\mspace{14mu} 2} \}\end{matrix}$

Consider that the thin layer corresponds to the thinned layer 2′: E₂, ν₂are, therefore, the Young's modulus and the Poisson's ratio,respectively, of the piezoelectric material that constitute the thinnedlayer 2′; h₂ is the intermediate thickness and r is the maximum radiusof the non-bonded areas present at the bonding interface 5 of the bondedstructure 6. The size of unbonded areas found at the bonding interface 5may, for example, be determined by imaging in white light or infraredaccording to the materials that are part of the bonded structure 6, orby acoustic microscopy, an especially advantageous technique fordetecting unglued areas of small size. The maximum radius of the areasnot bonded to the bonding interface 5 (which may be related to bondingdefects or to pre-existing patterns on one or the other of the assembledfaces of the donor and support substrates 1, 2 may thus be taken out foreach bonded structure 6.

The equation {eq. 2} indicates that the “buckling” phenomenon will beall the easier to initiate (i.e., it will require a lower normalizedforce F_(buckling) ^(norm.)) than the intermediate thickness h₂ of thethinned layer 2′ will be weak and that the maximum radius r ungluedareas will be large.

The support thickness h₁ of the support substrate 1 and its mechanicalcharacteristics (E₁, its Young's modulus) are also known; the requiredannealing temperature to be applied to the thinned structure 6′ duringthe heat treatment of step c) is also known. It is, therefore, possibleto express the force in normalized compression F^(norm.) that will applyto the thinned layer 2′ during the heat treatment of step c):

$\begin{matrix}{F^{{norm}.} = \frac{( {{E_{1} \cdot h_{1}^{3}} + {E_{2} \cdot h_{2}^{3}}} )}{6{( {h_{1} + h_{2}} ) \cdot \rho}}} & \{ {{eq}.\mspace{14mu} 3} \}\end{matrix}$

with ρ the radius of curvature of the thinned structure 6′:

$\begin{matrix}{\frac{1}{\rho} = \frac{\Delta\;{{CTE} \cdot \Delta}\; T}{K( {h\; 1{mh}\; 2} )}} & \{ {{eq}.\mspace{14mu} 4} \}\end{matrix}$with ΔCTE, the difference between thermal expansion coefficients of therespective materials of the thinned layer 2′ and the support substrate1; ΔT delta temperature between room temperature and annealingtemperature applied; and the term K (h₁, h₂) which is expressed as:

$\begin{matrix}{{K( {h_{1},h_{2}} )} = {\frac{h_{1} + h_{2}}{2} + {\frac{( {{E_{1} \cdot h_{1}^{3}} + {E_{2} \cdot h_{2}^{3}}} )}{6( {h_{1} + h_{2}} )} \cdot ( {\frac{1}{E_{1} \cdot h_{1}} + \frac{1}{E_{2} \cdot h_{2}}} )}}} & \{ {{eq}.\mspace{14mu} 5} \}\end{matrix}$

The threshold thickness of the range can thus be determined by solvingthe equation {eq. 5} F^(norm.)=F_(buckling) ^(norm.),

$\begin{matrix}{\frac{{( {{E_{1} \cdot h_{1}^{3}} + {E_{2} \cdot h_{2{thrsd}}^{3}}} ) \cdot \Delta}\;{{CTE} \cdot \Delta}\; T}{6{( {h_{1} + h_{2{thrsd}}} ) \cdot {K( {h_{1},h_{2{thrsd}}} )}}} = {\frac{\pi^{2}}{12}\frac{E_{2} \cdot h_{2{thrsd}}}{( {1 - v_{2}^{2}} )}( \frac{h_{2{thrsd}}}{r} )^{2}}} & \{ {{eq}.\mspace{14mu} 6} \}\end{matrix}$

with h_(2thrsd) the threshold thickness.

The threshold thickness corresponds to the intermediate thickness belowwhich the “buckling” phenomenon has a high probability of appearing,taking into account the characteristics of the thinned structure 6′(type of materials that constitute it, maximum size of unglued zonesfound at the bonding interface 5, thickness of the support substrate)and the annealing temperature to be applied during step c) heattreatment. Note that the “buckling” phenomenon can also be accompaniedby an enlargement of the unbonded zone, preferably perpendicular to thecrystallographic axes having the highest coefficient of expansion. Thisenlargement can all the more be as significant as the energy of thebonding interface 5 is low. Consolidating the energy of the bondinginterface 5, in a configuration to avoid the appearance of “buckling”is, therefore, of utmost importance.

The first sensitivity model thus connects the threshold thickness to aset of characteristic parameters of the thinned structure 6′ and to thethermal stress it must undergo.

The maximum thickness is determined from a second sensitivity modelwhose input parameters include the support thickness (of the supportsubstrate 1), the thermal expansion coefficients of the donor substrate2 and the support substrate 1 and the required annealing temperature instep c) heat treatment.

The second sensitivity model is established from an equation reflectingthe rupture of the material that constitutes support substrate 1,constrained in extension in the thinned structure 6′ during a heattreatment. Elastic energy E stored in the support substrate 1, in thecase of a thinned structure 6′ comprising a thinned layer 2′ and asupport substrate 1 of different nature and to which a heat treatment isapplied, is expressed as follows:

$\begin{matrix}{E = {{\frac{1}{E_{1}}( \frac{( F^{{norm}.} )^{2}}{h_{1}} )} + \frac{E_{2}^{2} \cdot h_{1}^{3}}{12 \cdot \rho^{2}}}} & \{ {{eq}.\mspace{14mu} 7} \}\end{matrix}$

Rupture of the material that constitutes support substrate 1 appearswhen elastic energy E exceeds a critical value given by:

$\begin{matrix}{E_{rupture} = \frac{K_{1c}^{2}}{E_{1}}} & \{ {{eq}.\mspace{14mu} 8} \}\end{matrix}$

with K_(1c) the tenacity of the material that constitutes supportsubstrate 1.

The maximum thickness of the range can thus be determined E=E_(rupture),by solving the equation:

$\begin{matrix}{( {{\frac{1}{h_{1}}\lbrack \frac{{( {{E_{1} \cdot h_{1}^{3}} + {E_{2} \cdot h_{2{ceiling}}^{3}}} ) \cdot \Delta}\;{{CTE} \cdot \Delta}\; T}{6{( {h_{1} + h_{2{ceiling}}} ) \cdot {K( {h_{1},h_{2{ceiling}}} )}}} \rbrack}^{2} + \frac{E_{1}^{2} \cdot h_{1}^{3} \cdot ( {\Delta\;{{CTE} \cdot \Delta}\; T} )^{2}}{12 \cdot \lbrack {K( {h_{1},h_{2{ceiling}}} )} \rbrack^{2}}} ) = K_{1c}^{2}} & \{ {{eq}.\mspace{14mu} 9} \}\end{matrix}$

with h_(2ceiling) the maximum thickness.

The maximum thickness corresponds to the intermediate thickness of thethinned layer 2′ above which the rupture of support substrate 1 has ahigh probability of appearing, taking into account the characteristicsof the thinned structure 6′ (type of material constituting it, thicknessof the support substrate 1) and the annealing temperature to be appliedduring the thermal treatment of step c).

The second sensitivity model thus connects the maximum thickness to aset of characteristic parameters of the thinned structure 6′ and to theheat stress it must undergo.

A heterogeneous structure such as thinned structure 6′ undergoes stressand deformations when subjected to a heat treatment, due to thedifferential expansions of the two materials that constitute it. Itdeforms by adopting a curvature that can lead to different failuremodes: the breaking of the support substrate 1, the buckling of thethinned layer 2′, the formation of dislocations or sliding planes in thestructure (especially in thinned layer 2′), the lift-off at the edges ofthinned structure 6′, etc. Experimentally, the Applicant realized that,in the case of thinned structures 6′ comprising a layer of piezoelectricmaterial and a support substrate of a material with a lower coefficientof thermal expansion, the two predominant failure modes were on the onehand the breakage of the support substrate 1 (rupture phenomenon of thematerial beyond the critical rupture energy) and secondly, the localbuckling at the level of non-bonded areas of the thinned layer 2′(phenomenon of relaxation of a thin layer in compression, called“buckling”). The two sensitivity models established by the Applicantfrom equations {eq. 6} and {eq. 9} are, therefore, well suited todetermining thickness ranges for the thinned layer 2′, in the case ofheterogeneous thinned structures 6′, the thinned layer 2′ of which ismade up of a piezoelectric material.

Following step a′), a thickness range defined by the determinedthreshold thickness and maximum thickness is thus obtained, asillustrated in FIGS. 2a to 2 c.

According to a first variant of implementing the manufacturing method,the effective thickness of the expected effective layer 20 for the finalhybrid structure 60 is less than the defined thickness range (FIG. 2a ):this is a case of thicknesses ranges that are compatible with theenvisaged final structure. The manufacturing method according to thedisclosure comprises, as mentioned above, step b) of thinning of thedonor substrate 2 to form the thinned layer 2′, after step a′) todetermine the thickness range with which it is compatible. Theintermediate thickness of thinned layer 2′ is then chosen to be in therange, i.e., between the threshold thickness and the maximum thickness.The process then comprises step c) of heat treatment at the annealingtemperature required for the thinned structure 6′. For instance, theannealing temperature may vary between 200° C. and 600° C. depending onthe type of hybrid structure expected and according to the aim of theheat treatment: consolidating bonding interface 5, cure of defects ordiffusion of light species in the thickness of thinned layer 2′(intended to become the effective layer 20), etc. Heat treatment maycomprise an inlet and an outlet of the oven at a low temperature, forexample, 100° C., a rise and fall ramp in temperature, for example,between 0.5°/minute and 5°/minute, and a bearing at the requiredannealing temperature, for example, between 200° C. and 600° C., for aperiod ranging from 30 minutes to a few hours.

After step c) of heat treatment, the method comprises a step d)corresponding to a second step of thinning the thinned layer 2′ to formthe effective layer 20 that has an effective thickness, disposed onsupport substrate 1; the whole forming the final hybrid structure 60(FIG. 1c ). Layer 2′ is thus thinned again, at its rear face 4, bytechniques of mechanical thinning, mechanical-chemical and/or chemicaletching and/or thinning by the SMART CUT® method. For instance, layer 2′can be thinned by mechanical-chemical polishing sequences, followed bycleaning. The hybrid structure 60 thus formed can then be used for theproduction of electronic devices, its properties (bonding energy ofbonding interface 5 and/or quality of effective layer 20) having beenimproved by the realization of the method according to the disclosure.Usually, the development steps of devices do not require heat treatmentsto be applied at a temperature as high as the annealing temperature ofstep c) of the method.

According to a second implementation variant of the manufacturingmethod, the effective thickness of the expected effective layer 20 forfinal hybrid structure 60 is within the defined thickness range (FIG. 2b); this is in a case of thickness range compatible with the envisagedfinal structure. The manufacturing method according to the disclosurecomprises, as mentioned above, step b) of thinning the donor substrate 2to form the thinned layer 2′, after step a′). The intermediate thicknessof the thinned layer 2′ is advantageously chosen to be in the range, andespecially, it is chosen to be equal to or substantially greater thanthe effective thickness. The process then comprises step c) of heattreatment at the annealing temperature required for the thinnedstructure 6′. For instance, the annealing temperature may vary between200° C. and 600° C. depending on the type of hybrid structure 60expected and according to the objective of the heat treatment.

After heat treatment in step c), the method comprises a step d)corresponding to a second step of thinning the thinned layer 2′ to formthe effective layer 20 with an effective thickness, disposed on supportsubstrate 1; the whole forming the final hybrid structure 60. Accordingto this second implementation variant, step b) of thinning the thinnedlayer already brings the intermediate thickness substantially to theeffective thickness. Step d) can thus consist essentially of a polishingstep with a low removal (“touch polishing” according to the Englishterminology) and cleaning sequences, to improve the surface condition ofthe face 4 of the effective layer 20. The hybrid structure 60 thusformed can then be used for the development of electronic devices,especially acoustic wave devices.

According to a third variant of implementing the manufacturing methodaccording to the disclosure, the effective thickness of the effectivelayer 20 expected for the final hybrid structure 60 is greater than thedefined thickness range (FIG. 2c ); in other words, the effectivethickness is greater than the maximum thickness. This configurationreflects the fact that the defined thickness range (determined by thesensitivity models and from the characteristics of the bonded structure6 and the required annealing temperature) is incompatible with theexpected final hybrid structure 60.

This thickness range configuration that is incompatible with theexpected hybrid structure (FIG. 2c ) can be as a result of theinadequacy of the support thickness. The manufacturing method, accordingto the disclosure, then comprises a step a″) of recycling the bondedstructure 6. Step a″) consists of achieving the lift-off of the bondedstructure 6 at the bonding interface 5, leading to the separation of thedonor substrate 2 and the support substrate 1. The separation can beperformed by inserting a bevel-shaped tool between the chamfered edgesof the two donor and support substrates 2, 1 of the bonded structure 6.After the separation, the recycling step a″) further comprises the reuseof the detached donor and support substrates 2, 1 for a new step a) toprovide a bonded structure 6. One can take advantage of the recyclingstep a″) to use a support substrate 1 of greater thickness and provide anew bonded structure 6. The increase in the support thickness notablyincreases the value of the maximum thickness, the aim of which is tofind a range of compatible thicknesses; i.e., with a maximum thicknessgreater than the expected effective thickness. The second thinning stepd) according to the method also comprises, in this case, a thinning stepof the rear face 4 of the support substrate 1, so as to bring it back tothe required support thickness for the final hybrid structure 60. Thisadditional thinning step may consist of a mechanical,mechanical-chemical or chemical thinning.

The configuration in which the thickness range is incompatible with theexpected hybrid structure (FIG. 2c ) may also be related to the factthat the annealing temperature is too high. One can then choose toreduce the annealing temperature to be applied in step c) of heattreatment. The recycling step a″) can also be used to apply a differentsurface preparation (potentially more complex or expensive but necessaryin this case) to the substrates 1 and 2 before their assembly, allowing,for example, promoting the bonding energy after a heat treatment at alower temperature. A new thickness range is then determined beforecontinuing the process.

According to a fourth implementation variant of the manufacturingmethod, the threshold thickness determined in step a′) is greater thanthe maximum thickness determined at the same step (configuration notshown). In this case, the thickness range is also consideredincompatible with the expected structure, since it does not exist (thethickness range being defined by a threshold thickness lower than amaximum thickness). It may be that in this case the maximum size of theunbonded areas found at the bonding interface 5 is too large to allow acompatible thickness range. The manufacturing method, according to thedisclosure, then comprises a step a″) of recycling the bonded structure6. Step a″) consists of achieving the lift-off of the bonded structure 6at the bonding interface 5, leading to the separation of the donorsubstrate 2 and the support substrate 1. The detachment can be done byapplying a stress at the interface between the two donor and supportsubstrates 2 and 1 of the bonded structure 6. After the separation, therecycling step a″) comprises the reuse of the detached donor substrates2 and support 1 for a new step a) of providing a bonded structure 6.Assuming that the unglued zone of maximum size was relative to a bondingfailure, the recycling step a″) can eliminate this defect by a newcleaning and preparation of the surfaces of substrates 1, 2 to beassembled.

If the size of the unbonded areas found at the bonding interface 5 isdifficult to reduce (in the case, for example, of pre-existing patternsor cavities on one of the two substrates assembled and having aparticular function), step a″) of recycling can be used, for example, touse and modify the assembly conditions in order to allow a reduction insubsequent temperature required to be applied in step c) and provide anew bonded structure 6.

The main steps of the method according to the disclosure are illustratedin FIG. 3.

The manufacturing method according to the disclosure makes it possibleto apply a heat treatment to an annealing temperature required toconsolidate the bonding interface 5 or to cure defects in the thinnedlayer 2′ (which becomes the effective layer 20), a thinned structure 6′for which the range of compatible thicknesses for the thinned layer 2′has been previously determined. The heat treatment is generally notapplicable to the final hybrid structure 60, i.e., with the effectivelayer 20 to its effective thickness, without generating damage to theeffective layer 20, especially when unglued areas (pre-existing bondingdefects or engraved patterns on the assembled faces of the substrates 1,2) are found at the bonding interface 5.

Besides, step a′) to determine the range of compatible thicknesses makesit possible to identify when it is necessary to recycle the bondedstructure 6. This is before engaging the thinning step b), which allowsthus to increase the manufacturing yields.

The disclosure also describes a hybrid structure 60 comprising aneffective layer 20 of piezoelectric material of effective thickness lessthan 50 microns assembled to a support substrate 1 having a coefficientof thermal expansion less than that of the effective layer 20 (FIG. 1c). The bonding interface 5 between the effective layer 20 and thesupport substrate 1 has a bonding energy greater than or equal to 1000mJ/m² and at least one non-bonded area whose size is between 1 and 1000microns. The effective layer 20 is composed of a material chosen fromthe group: lithium tantalate (LiTaO₃), lithium niobate (LiNbO₃),aluminum nitride (AlN), and zinc oxide (ZnO); for example, its thicknessis between 0.1 micron and 50 microns. The support substrate is made upof a material chosen from the group: silicon, III-V semiconductors,silicon carbide, glass, and sapphire; for example, its thickness isbetween 0.1 micron and 50 microns. The support substrate can alsocomprise one or more different types of surface layers. For example, thesupport substrate may consist of a single-crystal silicon substratehaving a surface layer on the side of its face to be assembled, capableof trapping charges, in particular, polycrystalline silicon; it may alsoconsist of an SOI substrate (silicon-on-insulator) whose surface layersare in silicon oxide and silicon or in an SOI substrate provided with acharge trapping layer under the oxide layer.

Example 1

A support substrate 1 made of silicon (Si) 150 mm in diameter, 725microns thick, has etched patterns spaced evenly over its entire face tobe assembled. These reasons may, for example, have a function ofalignment marks or constitute cavities for the manufacture of suspendedmembranes or more still made up of electrical contacts in the finalhybrid structure 60, on which the devices is developed. The supportsubstrate 1 also comprises an oxide layer on its face to be assembled.It is glued together by molecular bonding with a donor substrate 2 madeof lithium tantalate (LiTaO₃) of the same diameter to provide the bondedstructure 6. A microscopy acoustic control step makes it possible todetect and measure the non-bonded areas at the bonding interface 5,generated by the patterns. The maximum size of unglued areas correspondsto a radius r of 500 microns.

The expected final hybrid structure 60 has a useful 10 micron layer anda 725 micron support substrate. The annealing temperature to be appliedis 230° C. in order to sufficiently consolidate the bonding interface 5so that the hybrid structure 60 supports the subsequent steps ofdeveloping the acoustic wave devices.

Step a′) makes it possible to determine threshold and maximumthicknesses: h_(2threshold)=28 μm and h_(2ceiling)=32 μm. The thicknessrange obtained is compatible with the expected hybrid structure 60, theeffective thickness being less than the range.

Step b) thinning, consisting of mechanical thinning followed by chemicalmechanical polishing and chemical cleaning, allows the forming of athinned layer 2′ having an intermediate thickness of 30 microns. Theheat treatment of step c) is then carried out. Entry into the oven is at100° C., the ramp temperature rise is 1°/minute until it reaches a peakat 230° C., for a period of 4 hours. A temperature ramp down to1°/minute is then operated up to 100° C. before taking out the thinnedstructure 6′ of the oven. The thinned structure 6′ then undergoes asecond step of thinning the thinned layer 2′ to a thickness of 10microns to form the effective layer 20.

The hybrid structure 60 thus obtained is integral and has a consolidatedbonding interface 5 whose bonding energy is greater than or equal to1000 mJ/m², a useful integrated layer having no degradation linked to“buckling” phenomena and this despite the presence of unglued areas atits interface, a size ranging between 100 and 500 microns. Such a hybridstructure 60 can then be used for the development of acoustic wavedevices.

Example 2

A support substrate 1 made of silicon (Si) having a diameter of 150 mmand a thickness of 725 microns and comprising an oxide layer on its faceto be assembled is adhesively bonded to a donor substrate 2 made oflithium tantalate (LiTaO₃) of the same diameter to provide the gluedstructure 6. A microscopy acoustic control step makes it possible todetect two gluing defects (unglued areas) at the gluing interface 5,whose maximum size corresponds to a radius r of 700 microns.

The expected final hybrid structure 60 has a useful 10 micron layer anda 725 micron support substrate. The annealing temperature to be appliedis 250° C., with the aim of sufficiently consolidating the bondinginterface 5 so that the hybrid structure 60 supports the subsequentsteps of developing the acoustic wave devices.

Step a′) makes it possible to determine threshold and maximumthicknesses: h_(2threshold)=28 μm and h_(2ceiling)=25 μm. The thicknessrange obtained is not compatible with the expected hybrid structure 60,the threshold thickness being greater than the maximum thickness.

The recycling step a″) is then carried out, in order to reduce themaximum size of the gluing defects present at the gluing interface 5:the insertion of a tool in the form of a bevel at the level of thebonding interface 5 of the bonded structure 6 makes it possible toseparate the donor and support substrates 2, 1. A new cleaning andsurface activation sequence of the two substrates is carried out beforea new assembly to provide a new bonded structure 6. A new acousticmicroscopy control step makes it possible to detect ten defects at thebonding interface 5, the maximum size of which corresponds to a radius rof 150 microns.

On the basis of new characteristics of bonded structure 6, step a′)helps to determine the following threshold and maximum thicknesses:h_(2 threshold)=20 μm and h_(2 ceiling)=25 μm. The thickness rangeobtained is now compatible with the expected hybrid structure 60, thethreshold thickness being less than the maximum thickness and theeffective thickness being less than the range.

Step b) of thinning, consisting of a mechanical thinning followed bychemical mechanical polishing and chemical cleaning, allows a thinnedlayer 2′ whose intermediate thickness is 23 microns to be formed. Theheat treatment of step c) is then carried out. Entry into the oven isdone at 70° C., the ramp temperature rise ramp is 1°/minute until itreaches a plateau at 250° C., for a period of 4 hours. A temperatureramp down to 1°/minute is then operated up to 100° C. before removingthe structure from the oven. The thinned structure 6′ then undergoes asecond step of thinning the thinned layer 2′ to an effective thicknessof 10 microns to form the effective layer 20.

The hybrid structure 60 thus obtained is integral and has a consolidatedbonding interface 5 whose bonding energy is greater than or equal to1000 mJ/m, or even greater than 1500 mJ/m²; it also has a usefulintegral layer 20 with no degradation related to “buckling” phenomena,despite the presence of non-bonded areas at its interface of a sizebetween 50 and 150 microns. Such a hybrid structure 60 can then be usedfor the development of acoustic wave devices.

Of course, the invention is not limited to the embodiments and examplesdescribed, and variants can be provided without departing from the scopeof the invention as defined by the claims.

The invention claimed is:
 1. A method of manufacturing a hybridstructure having an effective layer of piezoelectric material with aneffective thickness disposed on a support substrate that has a supportthickness and a coefficient of thermal expansion less than that of theeffective layer, the method comprising: a) a step of providing a bondedstructure comprising a donor substrate of piezoelectric material and thesupport substrate; the bonded structure having a bonding interfacebetween the donor substrate and the support substrate; b) a first stepof thinning the donor substrate to form a thinned layer, having anintermediate thickness, disposed on the support substrate, the wholeforming a thinned structure; c) a heat treatment step of the thinnedstructure at an annealing temperature between 200° C. and 600° C.,including a rise or fall ramp in temperature between 0.5°/minute and5°/minute; d) a second thinning step, after step c), of the thinnedlayer to form the effective layer; wherein the method further comprises,prior to step b), a step a′) of determining a range of intermediatethicknesses of the thinned layer, the range being defined by a thresholdthickness and a maximum thickness, and wherein the intermediatethickness of the thinned layer is selected within this range, whereinthe threshold thickness is determined from a first sensitivity modelwhose input parameters include the support substrate thickness, thethermal expansion coefficients of the donor substrate and the supportsubstrate, the annealing temperature, and a maximum size of ungluedareas found at the bonding interface of the bonded structure.
 2. Themethod of claim 1, wherein the maximum thickness is determined from asecond sensitivity model whose input parameters include the supportthickness, the coefficients of thermal expansion of the donor substrateand the support substrate and the annealing temperature.
 3. The methodof claim 2, comprising after step a′) and prior to step b), a step a″)of recycling the bonded structure, when the determining step a′)establishes the threshold thickness higher than the maximum thickness orthe maximum thickness less than the effective thickness.
 4. The methodof manufacturing a hybrid structure according to claim 3, wherein thestep a″) of recycling further comprises a detachment at the bondinginterface of the bonded structure.
 5. The method of claim 4, wherein thestep a″) of recycling further comprises the reuse of the donor substrateand support substrate from the detachment structure for a new step a) toprovide a bonded structure.
 6. The method of claim 3, wherein the secondthinning step d) may further comprise a step of thinning the supportsubstrate.
 7. The method of claim 3, wherein the effective layer iscomposed of a material selected from among a group consisting of:lithium tantalate (LiTaO₃), lithium niobate (LiNbO₃), aluminum nitride(AlN), or zinc oxide (ZnO).
 8. The method of claim 3, wherein thesupport substrate is composed of a material selected from among a groupconsisting of: silicon, III-V semiconductor, silicon carbide, glass, orsapphire.
 9. The method of claim 8, wherein the support substratecomprises one or more surface layers.
 10. The method of claim 1,comprising after step a′) and prior to step b), a step a″) of recyclingthe bonded structure, when the determining step a′) establishes thethreshold thickness higher than the maximum thickness or the maximumthickness less than the effective thickness.
 11. The method of claim 1,wherein the second thinning step d) may further comprise a step ofthinning the support substrate.
 12. The method of claim 1, wherein theeffective layer is composed of a material selected from among a groupconsisting of: lithium tantalite (LiTaO₃), lithium niobate (LiNbO₃),aluminum nitride (AlN), or zinc oxide (ZnO).
 13. The method of claim 1,wherein the support substrate is composed of a material selected fromamong a group consisting of: silicon, III-V semiconductor, siliconcarbide, glass, or sapphire.
 14. The method of claim 13, wherein thesupport substrate comprises one or more surface layers.